Universities offer a range of educational courses and tuition for individuals aged 18 and over (although people can technically enter university at an earlier age in certain circumstances). In the past the UK education system managed higher education by both universities and polytechnics. Universities originally handled the more academic courses while polytechnics often took a more vocational approach. Nowadays the former polytechnics have either gained university status or have merged with older universities.
In most cases people will go to university to study as an undergraduate (i.e. to gain a degree of some sort) or as a post graduate (i.e. to study for a higher degree when they have achieved their first one). As well as the day to day tuition of degree level students many universities will also offer courses and options for the wider community from a learning perspective.
People choose the university that they want to attend and may then be accepted on a course or rejected. In many cases acceptance here will be based on existing A Level results or qualifications or on the A Level results that a student will get when they have sat their exams. University education in the UK is no longer free and students must pay a range of fees themselves in the majority of cases.